This invention relates to the field of force measuring apparatus and methods for determining the average stress in bolts or studs under tension. More particularly, this invention relates to a calibration stand and method for accurately calibrating an ultrasonic extensometer so that the readings obtained from the ultrasonic extensometer during normal use thereof can be relied upon as being accurate.
Ultrasonic extensometers are known in the art. The particularly preferred ultrasonic extensometer is shown and described in U.S. Pat. No. 3,759,090, which is incorporated herein by reference for all of its disclosure. Ultrasonic extensometers function by measuring the time difference required for ultrasonic signals to travel from one end of a bolt or stud to the other end and be reflected back. As a bolt or stud is stressed to tighten a joint, the bolt is elongated. That elongation of the bolt results in change in the time required for an ultrasonic signal to traverse the length of the bolt. Thus, by accurately monitoring the time required for passage of ultrasonic signals along the length of a bolt or stud, the tension load imposed on the bolt or stud can be determined from Hooke's law of linear relationship between stress and strain.
In order to be confident of the accuracy of readings from an ultrasonic extensometer, it is highly desirable to calibrate each extensometer before it is put into actual use, and it may be desirable to periodically recalibrate the extensometer to insure its continual accurate operation. To calibrate an extensometer, it becomes necessary to impose a tensile load on a bolt or stud, accurately determine both the level of the load and the change in length of the bolt or stud (the reference data) by accurate measuring apparatus other than an extensometer, simultaneously monitor the changes in length of the bolt or stud with an extensometer, compare the readings of the extensometer with the known reference values of force and strain, and adjust the output of the extensometer so that its output agrees with the reference data.
Accordingly, one object of the present invention is to provide novel and improved apparatus and methods for calibrating an extensometer.
Still another object of the present invention is to provide novel and improved apparatus and methods for obtaining reference data of force and strain while simultaneously monitoring the change in length of a bolt or stud under stress with an ultrasonic extensometer, and thereafter calibrating the ultrasonic extensometer to agree with the reference data.
Another object of the present invention is to provide novel and improved apparatus and methods for field testing of a bolt whereby both ends of the bolt are accessible for transducer and probe monitoring at all times when the bolt is under stress.
Other objects and advantages of the present invention will be apparent to and understood by those skilled in the art from the following detailed description and drawings.